For some years inorganic fibers containing alumina have been manufactured by melting mixtures containing granular alumina (or kaolin) and other oxides such as silica, allowing the molten material to be blown by gas or steam or to impinge on rotors turning at high speed, and causing the resulting blown or spun fibers to be accumulated on a collecting surface. These fibers are then used in bulk or in the form of mats, blankets and the like as high temperature thermal insulation. The compositions from which the melts are formed may contain up to about 80% of alumina. Commonly the principal other oxide present is silica. In the "glass" fibers the silica will be the major component and the alumina will be present in relatively small amounts. In the "aluminosilicate" ("refractory") fibers the alumina and silica contents are of the same general magnitude (i.e., each in the range of about 40% to 60% of the total composition), and in the "high alumina" fibers the alumina is the predominant material, being up to about 80% of the composition. (Unless otherwise stated, all percentages herein are by weight.) All of these fiber types (especially the glass fibers) may contain other oxides in addition to silica. In the aluminosilicate fibers the other oxides which are preset in various fiber compositions include chromia (U.S. Pat. No. 3,449,137 to Ekdahl), calcium and magnesium oxides or dolomite (U.S. Pat. No. 4,055,434 to Chen and Pallo) or titania, iron oxide, soda, zirconia and/or boria (U.S. Pat. No. 2,710,261 to McMullen). The fiber products will normally have service temperatures in the range of from about 600.degree. F. to 3200.degree. F. (315.degree. C. to 1760.degree. C.) depending on the particular oxide composition from which the fibers are formed; glass fibers are used in the lower ranges (e.g., 600.degree. F. to 1000.degree. F.; 315.degree. C. to 540.degree. C.), aluminosilicate fibers in the middle ranges (e.g., 800.degree. F. to 2700.degree. F.; 425.degree. C. to 1480` C.) and high alumina fibers in the higher ranges (e.g., 2300.degree. F. to 3200.degree. F.; 1260.degree. C. to 1760.degree. C.), although there is of course some overlap of service ranges and the exact limits of the range of each type of fiber are not absolute. Typical of commercial fibers are those refractory fibers sold under the trademarks "CERAWOOL," "CERAFIBER" and "CERACHROME" by Johns-Manville Corporation and also the glass fibers used by the same company in the manufacture of a wide variety of insulation products.
In the past, it was believed that in order to obtain sufficient refractoriness (i.e., ability to withstand appropriate service temperatures) of the fibers, it was necessary to use a highly calcined alumina as the alumina component in the melt from which the fibers are formed. Highly calcined aluminas have very high (usually 75% to 90%) contents of the alpha crystalline phase. These high alpha crystalline phase content aluminas have, however, has the disadvantage that they are quite slow melting. The production rate of fiber is therefore materially slowed because the formation rate of the melt is limited by the presence of the high alpha crystalline phase content alumina.
It would therefore be particularly desirable to have an oxide melt composition which would melt rapidly to allow increased fiber production rates while yet retaining the same desirable thermal properties of the fibers formed from such melt.